SIZING CRITERIA FOR RELIEF AND DEPRESSURING LINES

For preliminary engineering the guidelines given below should be followed. A preliminary network analysis will also be undertaken for all principal relieving/depressuring cases in order to ensure that the maximum allowable back pressure at each individual relieving device is not exceeded.

Calculations will be performed in accordance with API RP 520 and 521.

Relief Valve Inlet Lines

The inlet line pressure drop is to be less than 3% of set pressure calculated at set pressure conditions assuming flow based on installed relief valve area for conventional and balanced valves. (Note - not for pilots) However pressure drop in the inlet line for a pilot valve should be calculated and considered when determining the relief valve capacity. The inlet line shall not be smaller than the inlet flange of the relief valve.

Relief Valve Outlet Lines

The discharge line shall not be smaller than the outlet flange of the relief valve.  The computation of the allowable pressure drop using the required relief capacity is as follows:

Conventional Valves

Size the discharge line from conventional valves to limit the pressure drop to less than 10% of the set pressure (gauge).

Balanced Valves

Higher pressure drops may be used to affect considerable cost savings.  Limit the pressure drop to 50% of the set pressure (gauge) and to no more than the rating of the internal bellows.

Where there are a number of valves discharging into the same manifold, caution should be exercised to ensure that the back pressure in the manifold allows all relief valves to discharge properly.

The recommended maximum Mach number range is 0.5-0.75 calculated at rated flow rate and downstream end of header conditions.  Back pressure based on installed critical area shall be checked to confirm it is below the system design pressure.

Note that Mach number for vapor lines is calculated from the following equation:

vs =  ()^1/2

where   vs = sonic velocity  (m/s)

             R = individual gas constant = 8314/M (J/kg K) where

 M = gas molecular weight

 T = absolute temperature (K)

Flare Headers and Sub-Headers

The equivalent length of headers and sub-headers should be determined by reference to Crane Technical Paper No. 410. The maximum Mach number range is 0.5-0.75 calculated at rated flow rate at the downstream end of line conditions.

Flare Stacks

Flare Stack Diameter is generally sized on a velocity basis although pressure drop should be checked. For stable flare burning in pipe flares, API RP 521 recommends 0.5 Mach for a peak short term infrequent flow, with 0.2 Mach maintained for the more normal and possibly more frequent conditions.  Requirements for pipe flares and other proprietary designs should be discussed with the vendor.

Blowdown Inlet Lines

The maximum velocity should be limited to 200 ft/s.

Blowdown Outlet Lines and High Pressure Vents

Vent stack diameter is generally sized on a velocity basis although pressure drop should be checked.  Since there is no requirement to maintain a stable flame, the vent tip can be sized for sonic velocity.  This minimizes the size and provides gas dispersion.  An allowance must be made in the pressure drop calculation for the pressure discontinuity which occurs at the tip when sonic velocity is established.

Atmospheric Tank Vents

Non-refrigerated atmospheric tank vents are sized on the basis of maintaining an operating pressure that the tank can safely withstand.  Flow rates are determined by consideration of thermal inbreathing and out-breathing, maximum fluid inflow or outflow, and vapour production resulting from fire exposure.  Refer to References API RP- 2000 for design criteria.